Coating composition comprising chain-extendable crosslinkable polyol and diblocked diisocyanate diurethane oligomer

ABSTRACT

Resin composition suitable for high-solids solvent-based coating composition comprises chain-extendable, crosslinkable polyol of molecular weight from about 200 to 1000, having at least three hydroxyl groups, chain-extendable diblocked diisocyanate diurethane oligomer of molecular weight between about 300 to 1500, crosslinking agent reactive with the hydroxy functionality of the polyol and substantially unreactive with de-blocked isocyanate functionality of the diblocked diisocyanate diurethane oligomer, and, preferably, catalyst(s). The composition cures at elevated temperature to provide a coating on a substrate, such as steel, which coating is highly resistant to corrosion, humidity and solvents and provides corrosion protection for the substrate. The diblocked diisocyanate diurethane oligomer is preferably the reaction product of a diol with a half-blocked diisocyanate, wherein the half-blocked diisocyanate is preferably the reaction product of an organic diisocyanate with monofunctional blocking agent. The polyol is preferably the reaction product of a diol of molecular weight about 60 to 500 with a diepoxide of molecular weight about 100 to 1000 such as bisphenol. A epichlorohydrin epoxy resin, hydantoin epoxy resin and the like.

RELATED APPLICATIONS

This is a division of application Ser. No. 334,797 filed Dec. 28, 1981now U.S. Pat. No. 4,410,679.

INTRODUCTION

This invention relates to novel high solids, solvent-based,thermosetting resin coating compositions comprising chain-extendable,crosslinkable low molecular weight polyol, diblocked diisocyanatediurethane oligomer and crosslinking agent reactive with the polyol butsubstantially unreactive with isocyanate functionality. The compositionsare useful to make coatings which are highly resistant to corrosion,humidity and solvents.

BACKGROUND OF THE INVENTION

Solvent based coating compositions are known which employ high molecularweight (e.g. 2,000 to 10,000) polymer resins having crosslinkingfunctionality, and a suitable crosslinking agent. Typically, suchcoating compositions are applied to a substrate, for example, byspraying, and are then cured by baking the coated substrate at anelevated temperature suitable to drive off the organic solvent and topromote the crosslinking reaction. The resulting thermoset coating, ifsufficiently humidity and solvent resistant, can provide aesthetic andfunctional advantages including corrosion protection for the underlyingsubstrate.

Coating compositions comprising such high molecular weight polymerresins typically comprise only 25% to 50% solids so as to be sprayableor otherwise conveniently applicable to a substrate. The viscosity ofcoating compositions of higher solids content is typically too high forthis purpose. Conventional epoxy ester based automotive vehicle sprayprimers, for example, typically have a volatile organic content ("VOC")of approximately 540 g/l.

Elimination of the volatile organic solvent portion during curing ofthese conventional low-solids coating compositions presents toxicity andin some cases flammability hazards. Furthermore, bulk volume of thesecoating compositions is relatively large and therefore presentsundesirable material handling difficulties, and added expense.Furthermore, excessive solvent losses and/or solvent recovery equipmentadd considerable expense to the coating operation. Recently,governmental regulations on hydrocarbon emissions, particularlyapplicable to automotive coating operations, mandate a significantreduction in volatile organic content for coating compositions. Thus,for example, governmental guidelines for 1982 presently require thatemissions of volatile organics from automotive vehicle primer coatingcompositions be reduced to that equivalent to using coating compositionsof no greater than 350 g/l (2.9 lb./gal.) VOC. To meet governmentalguidelines, coating compositions of VOC greater than 350 g/l can beemployed in conjunction with emissions treatment equipment to achievethe specified emissions limit. Such treatment equipment presentssignificant additional expense, however, and thus there is a great needto provide coating compositions of VOC reduced near to, or preferablyeven lower than the 350 g/l governmental limit.

In response to these concerns, high solids coating compositions havebeen suggested which, typically, employ low molecular weightmulti-functional adducts or copolymers in combination withmulti-functional crosslinking agents. These high solids coatingcompositions are less viscous and, therefore, can be applied byspraying, for example, with far lower VOC than was possible withconventional epoxy ester based coating compositions or otherconventional coating compositions comprising high molecular weightpolymer resins. After application to the substrate, high solids coatingcompositions are cured by baking at a cure temperature, that is, at anelevated temperature suitable to drive off the volatile organic contentand to promote polymerization and crosslinking of the multi-functionallow molecular weight component(s).

Typically, these known high solids coating compositions yield curedcoatings having polymeric networks that differ significantly instructure and morphology from the polymeric networks provided byconventional, low solids coating compositions comprising high molecularweight polymers. Consequently, the physical properties of the coatingsprovided by such high solids coatings compositions can differsignificantly from those of the cured coatings provided by conventional,low solids coating compositions. In particular, the cured coatingsobtained from known high solids coating compositions can be inferior inthat they can be less flexible, less solvent resistant, less adherent tothe substrate and/or for other reasons provide less corrosion inhibitionfor the underlying substrate. Accordingly, it would be highly desirableto provide a coating composition comprising low molecular weightmaterials suitable for use in high solids, solvent based coatingcompositions and yet which, upon curing, form coatings having polymericnetworks similar in structure and morphology to those obtained withconventional low solids solvent-based coating compositions, and thushaving physical properties comparable to those obtained fromconventional low solids solvent based coating compositions.

Accordingly, it is an object of the present invention to provide novelhigh solids, solvent-based coating compositions. In this regard, it is aparticular object of the invention to provide novel coating compositionswhich are curable by chain-extension and crosslinking during cure, insitu, to form polymeric networks similar in structure and morphology tothose obtainable through use of conventional low solids, solvent-basedcoating compositions.

It is a particular object of the invention to provide a coatingcomposition of sufficiently low VOC to meet governmental guidelines. Itis also an object of the invention to provide such a coating compositionwhich can be applied to a substrate by spraying or other known method.

It is another object of the invention to provide a method of making acoating on a substrate, which coating has a polymeric network similar instructure and morphology to that provided by conventional low solidssolvent-based coating compositions and having similar advantageousphysical properties including, for example, humidity and solventresistance and corrosion protection for the underlying substrate.Additional aspects and advantages of the invention will be apparent fromthe following description thereof.

SUMMARY OF THE INVENTION

According to the invention, a high solids, organic solvent basedthermosetting resin composition comprises chain-extendable,crosslinkable low molecular weight polyol, having at least threehydroxyl groups, and chain-extendable low molecular weight diblockeddiisocyanate diurethane oligomer, and further comprises suitablecrosslinking agent such as, for example, aminoplast crosslinking agent,which is reactive with the polyol and is substantially unreactive withthe diblocked diisocyanate diurethane oligomer, in particular, withisocyanate functionality. The coating composition further comprisessuitable organic solvent such as, for example, butanol or other loweralkanol and, preferably, a catalyst for the crosslinking reaction and/orfor the chain-extension reaction. The diblocked diisocyanate diurethaneoligomer is of molecular weight between about 300 and about 1500,preferably between about 300 and about 800, and preferably is thereaction product of a half-blocked diisocyanate with a suitable diol. Ithas thermal de-blocking temperature of about 100° C. to 190° C.Preferred polyols are of molecular weight between about 200 and about1000, the more preferred being between about 200 and about 700, andinclude the reaction product of a suitable diol with a diepoxide ofmolecular weight between about 100 and about 1000.

According to another aspect of the invention, a method of making acorrosion, solvent and humidity resistant coating on a substratecomprises applying to the substrate the high-solids, solvent-basedthermosetting resin composition of the invention and heating the resincompositions to between about 100° C. and about 190° C. and preferablyto between about 130° C. and about 150° C. for a period sufficient toyield a cured coating.

The novel coating composition of the invention comprises latentinterreactive bifunctionality suitable for substantially linearchain-extension polymerization, in situ, on the surface of the substrateduring cure of the coating and further comprises hydroxy crosslinkingfunctionality. That is, the coating compositions of the invention form acoating on a substrate employing two different reactions, achain-extension polymerization reaction, in situ, to form high molecularweight hydroxy functional polyurethane and a crosslinking reactioninvolving said hydroxy functional polyurethane and a crosslinking agent.For reasons discussed further below, it is a significant aspect of theinvention that the crosslinking reaction is distinct from thechain-extension reaction.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, a high solids coating composition is one in which avolatile organic solvent content of about 400 g/l (3.4 lb./gal.) or lessyields a viscosity of less than approximately 35 sec. #4 Ford Cup at 27°C. (80° F.).

Preferred chain-extendable, crosslinkable, low molecular weight polyolssuitable for use in the coating composition of the invention comprisethree or more hydroxyl groups, preferably from 3 to 10 hydroxyl, suchthat upon chain-extension reaction with a de-blocked isocyanatefunctionality of each of two de-blocked diisocyanate molecules, duringcure, in situ, on the surface of the substrate, there remains at leastone additional unreacted hydroxy functionality for crosslinking reactionwith a suitable crosslinking agent such as, for example, an aminoplastcrosslinking agent. The polyol preferably has a molecular weight betweenabout 200 and abou 1000, and more preferably between about 300 and about700. Exemplary polyols suitable for the present invention includepolyhydroxy functional straight or branched chain saturated orunsaturated hydrocarbons, optionally comprising one or more oxy or estermoieties and optionally comprising one or more heterocyclic atoms,aromatic and/or heterocyclic rings, the heterocyclic atoms(s) beingselected preferably from N, O and S. Suitable polyol reactants includemany commercially available materials well known to the skilled of theart.

Preferred chain-extendable, crosslinkable polyols include epoxy-dioladducts, which can be provided as the reaction product of a suitablediepoxide with a suitable diol. Preferably the diepoxide (or each ofthem) has a number average molecular weight between about 100 and about1000, and more preferably between about 100 and about 700. Numerous suchpreferred diepoxides are readily commercially available, for example,Bisphenol A epichlorohydrin epoxy resins, for example, the Epon(trademark) series and the DER (trademark) series, Dow Chemical Company,Midland, Mich. Also preferred are cyclic aliphatic diepoxy resins, forexample, the Eponex (trademark) series of Shell Chemical Company,Houston, Tex., and hydantoin epoxy resins, for example, Resin XB2793(trademark) of Ciba-Geigy Corporation, Ardsely, N.Y. Preferreddiepoxides are terminal diepoxides, since these are generally morereactive and therefore require reaction conditions under whichundesirable side reactions, for example, epoxy-epoxy reactions andgellation, can be more easily avoided. Most preferred in view of theircommercial availability are Bisphenol A epichlorohydrin epoxy resins,for example, Epon 828 (trademark), Shell Chemical Co., Houston, Tex.Other, higher molecular weight members of the Epon (trademark) seriesare suitable to make higher molecular weight polyols which providecoating compositions of somewhat higher viscosity (or lower solidscontent). It should be recognized, however, that the higher molecularweight members of the Epon series, for example Epon 1001 and Epon 1004,may be somewhat less preferred, since the hydroxyl group(s) thereof canbe less sterically hindred and therefore more reactive. This can resultin undesirable side reactions, for example, reaction between the epoxyfunctionality and such hydroxy group of another diepoxide (rather thanwith an hydroxyl group of a diol). Also, however, improved properties,for example, improved corrosion resistance have been achieved withcoating compositions comprising polyols prepared using such materialsand the choice of suitable polyols (and of reactants for preparing same)will depend upon the particular application intended for the coatingcomposition. Also preferred are any of a wide variety of acyclic orcyclic aliphatic diepoxides such as, for example, 1,4-butanedioldiglycidyl ether and 4-vinylcyclexene dioxide and the like or a mixtureof any of them.

The diol suitable for preparing the polyol component of the coatingcomposition can be any of a wide variety of readily commerciallyavailable dihydroxy functional materials of which many are known to theskilled of the art. Preferred diols include those of molecular weightabout 60 to 500, more preferably about 60 to 200. Most preferred areterminal diols, that is, diols bearing two terminal hydroxyfunctionality, for example, 1,6-hexanediol since these are generallymore reactive. Other suitable aliphatic diols include primary/secondaryand secondary/secondary carbon hydroxy substituted diols. Diols bearingtertiary hydroxyl groups are least preferred due to their lowerreactivity. Thus, preferred diols include, for example alkyl substitutedor unsbustituted propanediol, butanediol, pentanediol, hexanediol, and amixture of any of them. Preferred aliphatic diols include, aliphaticdiols of about 2 to 20 carbons, for example, ethylene glycol,1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 2-ethyl-1,3-hexanedioland the like, or a compatible mixture of any of them. Other suitablealiphatic diols include, for example, ether diols, especially those of 4to about 20 carbons, for example triethylene glycol and the like.Suitable aromatic diols include those wherein one or both hydroxy groupsare substituted on a benzene ring. Preferrd aromatic diols comprise twohydroxyl groups substituted on the same benzene ring or on differentbenzene rings linked through a covalent bond or through one or morecarbons of a one to seven carbon, preferably three to five carbon,aliphatic moiety. Suitable aromatic diols include, for example,4,4'-isopropylidenediphenol (bisphenol A),4,4'-(1-methyl-propylidene)bisphenol (bisphenol B), catechol and thelike, or a compatible mixture of any of them.

The diepoxide is reacted with the diol according to methods well knownto the skilled of the art, preferably by slow addition to sufficientexcess of diol such that substantially each epoxide group reacts with anhydroxyl group of a different diol molecule. The resultant epoxy-dioladduct comprises four hydroxy groups: the one unreacted hydroxyl groupof each of the two diol molecules which reacted with the diepoxide, andthe hydroxyl group formed by each of the two cleaved epoxide rings.Employing a terminal diol and terminal diepoxide, the polyol reactionproduct has two terminal hydroxyls, each linked through a diol residueto the dihydroxy substituted diepoxide residue. Whether or not thepolyol comprises epoxy/diol adduct, as just described, it is preferredthat two of the three or more hydroxyls of the polyol be remote from oneanother and most preferred that they be terminal hydroxyls as definedabove.

The chain-extendable diblocked diisocyanate diurethane oligomer of thecoating composition preferably has a molecular weight between about 300and about 1500, more preferably between about 300 and about 800.According to a preferred embodiment of the invention, the diblockeddiisocyanate diurethane oligomer of the coating composition is preparedby reaction of a suitable half-blocked diisocyanate with suitable diol.It will be within the skill of the art, in view of the presentdisclosure, to select a suitable diol of which many, including aromaticand aliphatic diols are readily commercially available. Preferred diolsinclude those described above as being preferred for use in preparingthe polyol of the coating composition by reaction with diepoxide.

Suitable half-blocked diisocyanate is prepared by reaction of anysuitable organic diisocyanate with sufficient monofunctional blockingagent to block approximately one half of the isocyanate functionality.Suitable readily commercially available monofunctional blocking agentsare well known to the skilled of the art. The blocking agent is selectedsuch that the blocked isocyanate group will remain blocked for longperiods of time at normal storage temperatures but will be substantiallytotally "de-blocked" at elevated "cure" temperature. In addition, sincethe blocking agent will be released when the coating composition iscured by baking, it is preferred that the blocking agent have highvolatility near the de-blocking temperature and so will diffuse rapidlythrough the coating composition and evaporate completely therefromduring the baking step. Any blocking agent allowed to remain in thecured coating should be inert to the cured coating and to the substrateand to any other coatings to be used in conjunction therewith. It willbe within the skill of those skilled in the art, in view of the presentdisclosure, to select a suitable blocking agent to provide an unblockingtemperature meeting the requirements of each particular application ofth present invention. It will typically be preferred that the blockedisocyanate functionality be de-blocked (i.e., that the coatingcomposition be curable) at a temperature within the range of about 100°to about 190° C., more typically about 130° C. to about 150° C.Accordingly, preferred monofunctional blocking agents are selected fromamides, for example caprolactam, ketoximes, phenols and lower alcohols,for example alkanol of from one to about eight carbons, for example,methanol, ethanol, any propanol, any butanol, any pentanol, includingcyclopentanol, and the like, or a mixture of any of them. Approximatelyone molar equivalent of the monofunctional blocking agent is reactedwith the organic diisocyanate in a molar ratio of about 1:1. Suitabletechniques well known to the skilled of the art can be employed tomaximize the yield of half-blocked diisocyanate, such as, for example,adding the blocking agent slowly to the organic diisocyanate underreaction conditions.

Suitable organic diisocyanates are readily commercially available andinclude many known to the skilled of the art such as, for example,phenylene diisocyanates, toluene diisocyanates, isophoronediisocyanates, diisocyanatoalkane wherein the alkylene moiety has,preferably, from about three to ten carbons, for example,1,6-diisocyanatohexane, or the like or a compatible mixture of any ofthem. Most preferably the organic diisocyanate has a molecular weightless than about 250. If corrosion resistance is of primary concern inthe cured coating, for example in the case of an automotive vehicleprimer or topcoat, it may be preferred to use an aliphatic diisocyanate,for example, isophorone diisocyanate 1,6-hexane diisocyanate. Aromaticdiisocyanates provide suitable coatings, however, and may be preferredin view of their lower cost.

The half-blocked diisocyanate is then reacted with the previouslydescribed diol according to methods well known to the skilled of the artto produce the diblocked diisocyanate. That is, the unblocked isocyanategroup of each of two half-blocked diisocyanate molecules reacts with oneeach of the two hydroxyl groups of a diol molecule. Each produces aurethane linkage. The reaction product comprises two blocked isocyanategroups. During cure, each blocked isocyanate group is de-blocked andwill undergo chain-extension reaction with an hydroxy group of thepolyol component of the coating composition. The half-blockeddiisocyanate and diol are reacted in molar ratio of about 2:1,respectively.

The coating composition of the invention comprises latent interreactivebifunctionality suitable for substantially linear chain-extensionpolymerization, namely, the hydroxy functionality of the polyol and theblocked isocyanate functionality of the diblocked diisocyanatediurethane oligomer, and further comprises hydroxy functionalitysuitable for crosslinking reaction separate and distinct from thechain-extension reaction. It is preferred that the blocked isocyanategroups and two of the three or more hydroxyl groups of the polyol eachbe an end group. Reactions between such hydroxyl end groups and blockedisocyanate end groups are believed to provide most efficientchain-extension during cure.

The molecular weight of the polyol and of the diblocked diisocyanatediurethane oligomer will affect the volatile organic content needed inthe coating composition comprising same. Where a high-solids coatingcomposition is desired, preferably the molecular weight of each iswithin the ranges specified above, since this has been found to providehigh-solids resin compositions according to the invention which can beapplied to a substrate by spray or other means in a coating compositionhaving a volatile organic content as low as about 350 to 400 g/l (2.9 to3.4 lb./gal.) or less.

Suitable crosslinking agent is that which will react with the hydroxyfunctionality of the long chain polymer product of the chain-extensionpolymerization reaction between the polyol and the de-blockeddiisocyanate diurethane oligomer. Suitable crosslinking agents will notreact, however, with the de-blocked isocyanate groups. Accordingly,crosslinking reaction in the preferred coating composition is a reactionseparate and distinct from the hydroxy-isocyanate chain-extensionreaction. Numerous such crosslinking agents are well known to theskilled of the art and include, for example, any of a variety ofaminoplast crosslinking agents, for example, partially alkylatedmelamines (melamine formaldehyde resins modified by alcohols), forexample partially methylated melamines and butylated melamines;polyalkyl ethers of the polymethylol melamines, for example hexamethoxymethylmelamine; urea formaldehyde condensate modified by alcohol, forexample butylated urea resin; polymerides of formaldehyde, for example,paraformaldehyde and trioxane; polymethylol compounds of hexamethylenediurea; adipic acid dimethylol amide and methylol ether thereof;tetramethylolhydrazodicarbonamide; polymethylol compounds ofpolycaprolactam and methylol ethers thereof; and the like or compatiblemixtures of any of them. Other suitable crosslinking agents will beapparent to the skilled of the art in view of the present disclosure.Hexamethoxymethyl melamine is preferred since it is readily commerciallyavailable, is of low molecular weight and has been found to providesuitable crosslinking activity.

The oligomer and polyol of the invention are used preferably in molarratio of about 1:1 to about 1:3, more preferably about 1:1 to about1:1.2, respectively. The proper proportion of crosslinking agent in thecoating composition will depend upon the properties desired in thecoating to be produced. Generally a somewhat less than stoichiometricamount of crosslinking agent can be used to provide a cured coating ofgreater flexibility. Where hexamethyoxymethyl melamine or the like isemployed with a preferred polyol and diblocked diisocyanate diurethaneoligomer, described above, a generally preferred weight ratio ofcrosslinking agent to polyol is about from 1:1 to 1:15, more preferredbeing about 1:1 to about 1:5, respectively. Too much crosslinking agentcan produce a coating which is brittle and humidity sensitive. If toolittle is used, the coating will not cure properly.

It will be within the skill of the art to determine the proper volatileorganic content for a given coating composition of the invention, for agiven application. In general, preferred solvents are those having aboiling point between about 60° C. and about 200° C., more preferablybetween about 110° C. and about 170° C. Preferred solvents include, forexample, butanol, methyl amyl ketone and the like, or a mixture thereofsuch as a 1:2 mixture of butanol and methyl amyl ketone, respectively,which is generally preferred for coating compositions intended forautomotive vehicle coating operations and the like. Additional suitablesolvents will be apparent to the skilled of the art in view of thepresent disclosure.

Any solvent allowed to remain in the cured coating must be inert so asto avoid adverse effect upon the cured coating or upon another coatingused in conjunction with it, during the curing process or thereafter.Preferrably, the cured coating is completely free of solvent. Thepreferred solvents, in addition have relatively low volatility attemperatures appreciably below their boiling points such that solventevaporation is low during storage and/or application of the coatingcomposition to the substrate.

Sufficient solvent is used to reduce the viscosity of the coatingcomposition to a level suitable for application to the substrate in thedesired manner. While conventional epoxy ester-type automotivespray-applied primer coating compositions are known to require avolatile organic content of about 540 g/l, the novel coatingcompositions of the present invention require as little as 350-400 g/lor less VOC to provide a spray viscosity of 25.35 sec, #4 Ford Cup. Ofcourse, the coating compositions of the invention need not be formulatedas a "high solids" composition, but rather can have a higher VOC toprovide a lower viscosity. It is generally preferred that sufficientsolvent be used to provide a viscosity of about 15 to 35 seconds, No. 4Ford Cup at 27° C. (80° F.).

Also preferably included in the coating composition of the invention isany of a variety of acid catalysts known to the skilled of the art tocatalyze the aminoplast crosslinking reaction, for example,p-toluenesulfonic acid, phosphoric acid, phenyl acid phosphate, butylphosphate, butyl maleate, and the like or a compatible mixture of any ofthem. In addition, any of a variety of catalysts for the isocyanatede-blocking reaction can also be included in the coating composition,for example, dibutyl tin dilaurate. In addition, a flow control agent,for example, polybutyl acrylate; a wetting agent, for example, silicone;pigment(s); a pigment dispersent; and/or a corrosion inhibitor, forexample, chromate pigment, numerous of all of which are known to theskilled of the art, may be employed in the coating compositions of theinvention.

It should be recognized that the coating compositions can comprise adiol in addition to the polyol. The diol would contribute nocrosslinking functionality to the high molecular weight chain-extendedpolymerization product of the cured coating. That is, the diol wouldprovide two hydroxy for chain-extension reaction with de-blockedisocyanate functionality, but would provide no additional hydroxyls onthe polymerization product for crosslinking reaction with thecrosslinking agent. By simple adjustment of the proportion of diol topolyol in the coating composition, the crosslink density in the curedcoating, and therefore the degree of flexibility of the cured coatingcan be accurately controlled. Readily commercially available diolssuitable for use in the present invention will be apparent to theskilled of the art in view of the present disclosure. Preferred diolsinclude those described above as being preferred for use in preparingthe polyol by reaction with diepoxide.

According to another aspect of the invention, a coating on a substrateis provided, which coating comprises the chain-extended, crosslinkedpolymer product following cure of a coating comprising the coatingcomposition of the invention. The coating composition can be a lowsolids composition, that is, it can have a high VOC, but generally ahigh solids composition, that is, one having a low VOC is preferred forthe reasons given above. It can be applied by any conventional method,including brushing, dipping, flow coating, spraying, etc. Spraying willgenerally be preferred, for example, for applying the composition as anautomotive primer or topcoat. In such spraying applications, the coatingcompositions of the invention are especially advantageous for use ashigh solids compositions. In this regard, coating compositions of theinvention employing preferred polyol, diblocked diisocyanate diurethaneoligomer, crosslinking agent and solvent, as described above, aresuitable to be applied to a substrate by spraying even though formulatedat volatile organic content levels as low as about 330 to 360 g/l (2.7to 3.0 lb/gal).

Curing the coating composition requires baking for sufficient time atsufficiently elevated temperature to de-block the blocked isocyanatefunctionality of the diblocked diisocyanate diurethane oligomer. Thetime and temperature required to cure the coating are interrelated anddepend upon the particular polyol, diblocked diisocyanate diurethaneoligomer, crosslinking agent, solvent and other materials, if any, andthe amount of each comprising the coating composition. Employing avolatile organic content of about 350 g/l and selecting preferredcomponents as described above, the required bake time and temperature istypically about 20 to 30 minutes and about 180° C. The temperaturerequired for cure can be reduced to about 150° C. for 20 to 30 minutesby addition of suitable catalyst such as any of those known to theskilled of the art, for example, dibutyl tin dilaurate.

High solids coating compositions according to the present invention,comprising the low molecular weight chain-extendable, crosslinkablepolyol, especially the preferred epoxy/diol adducts described above, lowmolecular weight chain-extendable diblocked diisocyanate diurethaneoligomer and, preferably, an aminoplast crosslinking agent, for example,hexamethoxymethyl melamine, have been found to afford cured coatingswith corrosion inhibition properties comparable to conventional epoxyester based, low solids sprayable coating compositions. The significantreduction in volatile organic content made possible by the coatingcompositions of the invention is, therefore, a highly advantageousadvance in the art.

As it is presently understood, chain-extension reactions by each of twohydroxyls of the polyol molecules with an isocyanate functionality ofdifferent diblocked molecules, which are de-blocked at curetemperatures, provides substantially linear chain-extension, in situ, onthe surface of the substrate during cure of the coating composition. Theadditional hydroxy functionality of the polyol not undergoingchain-extension reaction is available for crosslinking reaction with thecrosslinking agent. While not wishing to be bound by theory, it ispresently understood that upon curing a coating composition according tothe present invention, the blocked isocyanate group is de-blocked andreacts more readily with the least sterically hindered of the availablehydroxyl groups. If the polyol comprises a terminal hydroxyl group, asin preferred embodiments described above, then the de-blocked isocyanategroup would react most readily with such terminal hydroxyl group and notwith any of the one or more additional, non-terminal hydroxyl group(s)of the polyol (or of the extended chain polymer being formed by thecuring process). Such additional non-terminal hydroxyl functionalityremains available for crosslinking reaction. If, for example, the polyolemployed is an epoxy/diol adduct reaction product of a terminal diol,for example, 1,6-hexanediol, with a terminal diepoxide, for example, anhydantoin epoxy resin, then the polyol will have two terminal hydroxyland two non-terminal hydroxyls (formed by cleavage of the epoxiderings). During cure, according to present understanding, the de-blockedisocyanate functionality will react predominantly with the terminalhydroxy. The result is substantially linear chain-extensionpolymerization, in situ, on the surface of the substrate, to form longchain, high molecular weight polymers with pendant hydroxyl groupsavailable for crosslinking reaction. Accordingly, the polymer networksobtained during cure of the coating compositions of the presentinvention are believed to be similar in structure to those obtainedusing conventional low solids solvent based coating compositions. Thisresult is also indicated by the improved physical properties, especiallyby the improved flexibility provided by the cured coating in comparisonto that which has been achieved with conventional high molecular weightcoating compositions.

Even where an isocyanate group does not react with a terminal hydroxylgroup, however, the isocyanate groups can react only with the polyol,since they are not reactive with the crosslinking agent, and the resultis substantially linear chain-extension polymerization.

In addition, network crosslink density can be controlled, and thereforethe flexibility of the cured coating can to a large extent be controlledby proper selection of the polyol. Crosslink density increases andflexibility decreases as the hydroxy functionality is increased and/oras the molecular weight of the polyol and/or of the diblockeddiisocyanate diurethane oligomer is reduced. Thus, it will be apparentto the skilled of the art that if the polyol and diblocked diisocyanatediurethane oligomer are prepared according to the preferred embodimentsdescribed above, then the selection of the diepoxide, diol, and organicdiisocyanate reactants provides substantial control of the crosslinkdensity in the cured coating. The greater the molecular weight of thereactants, the lower will be the crosslink density in the cured coating.Thus, for example, where the polyol is the reaction product of adiepoxide and a diol, there will be a higher crosslink density in thecured coating if the diol is 1,3-propanediol than if it is1,6-hexanediol.

In addition, it will be recognized by the skilled of the art in view ofthe present disclosure that higher molecular weight components in thecoating composition will, in general, provide a more viscous coating ata given VOC. Higher molecular weight polyol and diblocked diisocyanatediurethane oligomer are for that reason less preferred where a highsolids coating composition is desired.

Cured coatings according to the invention have been found to provideexcellent corrosion resistance when applied over a metallic substratesuch as, for example, when applied as an automotive vehicle primer coatover bare sheet steel. While not wishing to bound by theory, theexceptional corrosion inhibitors provided by preferred embodimentsdescribed above stem, in part, from the absence of ester linkages. Esterlinkages are known to be attacked by hydroxide, a product of the metalcorrosion process.

EXAMPLE I Preparation of Epoxy/Diol Adduct Polyol

This example illustrates the preparation of a polyol suitable for use inthe coating composition of the invention. More specifically, itillustrates the preparation of an epoxy/diol adduct from a heterocyclicepoxy resin XB2793 (trademark, Ciba-Geigy Corporation), 138. g,1,3-hexanediol, 146. g, and N,N-dimethylethanolamine, 0.5 g, arecombined in methyl amyl ketone, 71. g, and refluxed approximately 40hours until the epoxy infrared absorption disappears. The low viscosityresin product is cooled to room temperature and stored.

EXAMPLE II Preparation of Epoxy/Diol Adduct Polyol

The example illustrates the preparation of an epoxy/diol adduct from aheterocyclic epoxy and an aromatic diol. Hydantoin epoxy resin XB2793(trademark, Ciba-Geigy Corporation), 69. g, and Bisphenol A, 114. g, arecombined in methyl amyl ketone, 45.8 g, and refluxed approximately 4hours until the epoxy infrared absorption disappears. The low viscosityresin product is cooled to room temperature and stored.

EXAMPLE III A. Preparation of Half-blocked Diisocyanate

This example illustrates the preparation of a diblocked diisocyanatesuitable for use in the coating composition of the present invention. Analcohol half-blocked aliphatic diisocyanate is prepared. Butyl alcohol,18.3 g is added dropwise to a mixture of isophorone diisocyanate, 54.7g, and dibutyl tin dilaurate, 0.3 g in methyl amyl ketone, 18.3 g. Afteraddition of the alcohol, the mixture is heated to between 60°-80° C. for2 hours. (Higher temperatures were avoided to avoid undesirable sidereactions.) The half-blocked diisocyanate product is characterized byinfrared spectroscopy showing the absence of OH absorption at 3300 cm⁻¹and 1730 cm⁻¹, respectively.

B. Preparation of Diblocked Diisocyanate Diurethane Oligomer

The half-blocked diisocyanate of Part A is reacted2-ethyl-1,3-hexanediol, 18.2 g, in 22.8 g methyl amyl ketone, at 60°-80°C. until no infrared absorption for isocyanate ias observed. The productis stored at room temperature.

EXAMPLE IV Preparation of Epoxy/Diol Adduct Polyol

An aromatic epoxy/branched chain aliphatic diol adduct is prepared byrefluxing a mixture of the Bisphenol A epichlorohydrin epoxy resin Epon828 (trademark, Shell Chemical Company), 190. g, 2-ethyl-1,3-hexanediol,146. g, methyl amyl ketone, 84. g, and N,N-dimethylethanol amine, 0.5 g,for about 4 to 8 hours until the infrared epoxide absorption disappears.

EXAMPLE V A. Preparation of Half-blocked Diisocyanate

The example illustrates the preparation of a diblocked diisocyanatesuitable for use in the coating composition of the present invention.This Part A illustrates the preparation of an alcohol half-blockedaromatic diisocyanate. Butanol, 18.3 g, is added dropwise, withstirring, to toluene diisocyanate, 43.1 g, and dibutyl tin dilaurate,0.3 g in 18.3 g of methyl amyl ketone. The reaction temperature ismaintained at about 60°-80° C. for two hours. The half-blockeddiisocyanate product is verified by infrared spectroscopy as in ExampleIII. The product is used in Step B.

B. Preparation of Diblocked Diisocyanate Diurethane Oligomer

The half-blocked diisocyanate product of Part A is reacted with2-ethyl-1,3-hexanediol, 18.2 g, in 22.8 g methyl amyl ketone at 60°-80°C. until no infrared absorption for isocyanate is observed.

EXAMPLE VI Preparation of Epoxy/Diol Adduct Polyol

This example illustrates the preparation of an epoxy/diol adduct from anaromatic epoxy and a straight chain aliphatic diol. Epon 828 (trademark,Shell Chemical Company, 190. g, 1,5-pentanediol, 78. g, anddimethylethanolamine, 0.68 g, are combined in methyl amyl ketone, 67. g.The reaction mixture is heated at 100°-130° C. for 16 hours. Theproduct, under infrared spectroscopy, reveals no absorption for epoxy.The product is stored for later use.

EXAMPLE VII Preparation of Epoxy/Diol Adduct Polyol

This example illustrates the preparation of an epoxy/diol adduct from analiphatic epoxy and a branched chain aliphatic diol. The Bisphenol Aepichlorohydrin epoxy resin Eponex 151 (trademark, Shell ChemicalCompany), 234. g, 2-ethyl-1,3-hexanediol, 146. g, andN,N-dimethylethanol amine, 1. g are combined and heated at 120°-140° C.for about 20 hours. The product, under infrared spectroscopy, showed noabsorption for epoxy. The product is stored at room temperature.

EXAMPLE VIII Preparation of Epoxy/Diol Adduct Polyol

This example illustrates the preparation of an aliphatic epoxy/aliphaticdiol adduct. The Bisphenol A epichlorohydrin epoxy resin Eponex 151(trademark, Shell Chemical Company), 234. g, 1,5-pentanediol, 104. g,and N,N-dimethylethanol amine are heated at 120°-140° C. for about 20hours. The product, under infrared spectroscopy, showed no absorptionfor epoxy. The product is stored at room temperature.

EXAMPLE IX Preparation of Coating Composition and Cured Coating

A typical coating composition according to the invention was prepared asfollows. Diblocked diisocyanate diurethane oligomer prepared accordingto the procedure of Example III, 114.1 g, is combined with polyolconsisting of epoxy/diol adduct prepared according to the procedurePG,25 of Example VI, 66.5 g, hexamethoxymethylmelamine, 38. g, andparatoluene sulfonic acid, 1. g, in 20. g, methyl ethyl ketone. Bare,unpolished steel panels are coated with the above composition and bakedat 180° C. for 20 minutes. The cured coating is found to have goodsolvent, humidity and corrosion resistance.

EXAMPLE X Preparation of Coating Composition and Cured Coating

A pigment coating composition according to the invention is preparedconsisting of the pigments and binder package listed below.

    ______________________________________                                        Pigment Package                                                               Grams          Pigment                                                        ______________________________________                                        4.3            silica                                                         48.4           barytes                                                        0.6            carbon black                                                   6.5            titanium dioxide                                               ______________________________________                                        Binder Package                                                                Grams                                                                         ______________________________________                                        114.1          diblocked diisocyanate                                                        diurethane oligomer                                            66.5           polyol                                                         38.0           hexamethoxymethylmelamine                                      1.0            para-toluene sulfonic acid                                     30.0           1:2 butyanol/methyl amyl                                                      ketone                                                         ______________________________________                                    

The diblocked diisocyanate diurethane oligomer is that preparedaccording to the method of Example III. The polyol consists of anepoxy/diol adduct prepared according to the method of Example VI. Bare,unpolished steel panels are coated with the above composition and bakedat 180° C. for 20 minutes. The cured coating is found to have goodsolvent, humidity and corrosion resistance.

Particular embodiments of the present invention described above areillustrative only and do not limit the scope of the invention. It wil beapparent to the skilled of the art in view of the foregoing disclosurethat modifications and substitutions can be made without departing fromthe scope of the invention.

We claim:
 1. A resin composition comprising:chain-extendable,crosslinking polyol of molecular weight about 200 to about 1000, havingat least three hydroxyl groups, selected from polyhydroxy functionalstraight or branched chain saturated or unsaturated hydrocarbons,optionally comprising one or more oxy or ester moieties and optionallycomprising one or more heterocyclic atoms, aromatic and/or heterocyclicrings, the heterocyclic atom(s) being selected from N, O and S, whereinsaid chain-extendable crosslinkable polyol is the reaction product of adiepoxide of molecular weight about 100 to 1000 with a diol of molecularweight about 60 to 500; chain-extendable, diblocked diisocyanatediurethane oligomer reaction product of diol with a half-blockeddiisocyanate in molar ration of about 1:2, respectively, said oligomerbeing of molecular weight about 300 to about 1500, the blocking group ofwhich oligomer has a de-blocking temperature of about 100° C. to about190° C.; and crosslinking agent reactive with the hydroxy functionalityof said polyol and substantially unreactive with de-blocked isocyanatefunctionality of said diblocked diisocyanate diurethane oligomer,wherein said crosslinking agent and said polyol are present in a molarequivalent ratio of about 1:1 to about 1:15, respectively.
 2. The resincomposition of claim 1, wherein said diepoxide has a molecular weight ofabout 100 to
 700. 3. The resin composition of claim 1, wherein saiddiepoxide bears two terminal epoxide functionality.
 4. The resincomposition of claim 1, wherein said diepoxide is selected from thegroup consisting of Bisphenol A epichlorohydrin epoxy resin, hydantoinepoxy resin, cyclic or acyclic aliphatic diepoxide and a mixture of anyof them.
 5. The resin composition of claim 1, wherein said diol hasmolecular weight of about 60 and
 200. 6. The resin composition of claim1, wherein said diol is selected from the group consistion of alkylsubstituted or unsubstituted propanediol, butanediol, pentanediol,hexanediol, and a mixture of any of them.
 7. The resin composition ofclaim 1, wherein said diol bears two terminal hydroxy functionality. 8.The resin composition of claim 1, wherein said diol comprises aromaticdiol wherein each hydroxyl group is substituted on the same benzene ringor on different benzene rings linked through a covalent bond or throughone or more carbons of a one to seven carbon aliphatic moiety.
 9. Theresin composition of claim 8, wherein said diol is selected from thegroup consistion of Bisphenol A, Bisphenol B, catechol and a mixture ofany of them.